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Pension and Other Postretirement Benefit Plans - Components of Net Periodic Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pension      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Service Cost $ 6.5 $ 9.3 $ 11.0
Interest Cost 40.5 27.2 24.6
Expected Return on Plan Assets (43.8) (41.5) (43.4)
Amortization of (Gain) Loss 5.8 11.4 18.8
Amortization of Prior Service Credit (0.1) (0.1) (0.2)
Net Pension Cost 8.9 6.3 10.8
Postretirement Health and Life Obligation and Funded Status      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Service Cost 2.0 3.0 3.6
Interest Cost 5.6 4.4 4.4
Expected Return on Plan Assets (11.4) (9.6) (9.9)
Amortization of (Gain) Loss (2.7) 0.4 3.0
Amortization of Prior Service Credit (8.5) (7.5) (7.6)
Net Pension Cost $ (15.0) $ (9.3) $ (6.5)